#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
int find(int i)
{
	int arr[] = { 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20 };
	int left = 0;
	size_t right = sizeof(arr) / 4 - 1;
	while (left <= right)
	{
		int ave = (left + right) / 2;
		if (i < arr[ave])
		{
			right = ave - 1;
		}
		else if (i > arr[ave])
		{
			left = ave + 1;
		}
		else
		{
			return ave;
		}
	}
	if (left > right)
	{
		return EOF;
	}
}